L Bu state

Electroabsorption (Martin et al. 1999), and two-photon absorption and photoinduced absorption (Frolov et al. 2002) indicate a dipole forbidden state at ca. 0.7 eV higher in energy than the l Bu state in PPV derivatives. This state is labelled the m Ag state, and is indicated in Fig. 11.1(b). Electroabsorption in PFO indicates that the m Ag state is 0.8 eV higher in energy than the l Bu state (Cadby et al. 2000). 

Phosphorescence indicates a triplet state at ca. 0.7 eV lower in energy than the l Bu state for a wide variety of systems. (See Kohler and Beljonne (2004) for a review of the data.) This triplet state is the 1 B state. 

As is the case for the Si (2 Ag) state, the vibronic development of the S2 (l Bu) state is dominated by the C-C and C=C stretch modes. Table 5 summarizes values for these frequencies in the S2 excited state for a number of linear polyenes. There is a general trend to lower frequency with increasing chain length, particularly for the C=C stretch frequency. As is also the case in the ground and Si states, the C-C mode depends on substitution and often appears as a complex multiplet. 

Relative to the question of whether the (l) Bu and (2)1 Ag state cross and, if so, when, the authors believed crossing might occur between n = 9 and n = 11 (96JPC18683). 

The time-scale for the screening is determined by the time to establish a dipole in the dielectric, namely h/AE l Bu), and the time for the screened molecule to make a transition to a different state, namely h/ AE Xf) — AE Xi)). As the excited state energies increase the energy differences decrease and this time scale diverges. Eventually this leads to a break-down in the perturbation theory, and to solvation-like screening, as in a point charge described below. 

Figure 10.2 illustrates the electroabsorption spectrum of phenyl-substituted traras-polyacetylene thin film (Liess et al. 1997). The feature at 2.0 eV is the red-shifted l Bu exciton. The feature at 2.5 eV is attributed to a dipole-forbidden state, namely the m Ag state. Unlike polydiacetylene crystals, disordered trans-polyacetylene thin film does not exhibit Pranz-Keldysh oscillations (described in Chapter 8) and therefore a definite assignment of a conduction band edge cannot made. However, because disordered polydiacetylene also does not exhibit Pranz-Keldysh oscillations, but a smeared-out feature similar to the one exhibited at 2.5 eV in Fig. 10.2 it is sometimes assumed that this feature does mark the band edge. Another interpretation is that this feature represents the n = 2 Mott-Hubbard exciton, described in Chapter 6, with the particle-hole continuum lying close in energy (possibly at 2.7 eV, which is three times the THG feature at... 

The most complex of all considered spectra is H(D20), shown in Fig. 5.3c. Here the deuteration of the external positions brings the position of the zeroth-order state lower frequencies by about 300cm , while the zeroth-order states l u) and I u.ir) remain almost unaffected. This results in a situation in which the zeroth-order zeroth-order states strongly couple to each other and are responsible for the triplet absorption with peaks at 938,1,355, and 1,564 cm [12,40]. The peak at 938 cm has almost equal relative contribution from l i ) and ). The central peak at l,355cm has almost equal participation from bu), and while... 

Figure 9. Excess 7C-electron density (7C-electron densities minus 1) in the 2 Ag state of hexadecaoctaene that follow from a fit of the polyene 2iAg and l Bu excitation energies by the simple model described in Section 4. The net charge on a carbon atom is the negative of the excess 7t-electron density.

Because most fuel ethanol manufactured ia the United States is made from com, price plays a cmcial role ia determining the competitive position of ethanol ia an open market. With com priced at about 2.50/bu, the embedded feedstock cost of product ethanol is about 0.14—0.23/L ( 0.52—0.87 gal), depending on over-all yield and by-products ignored (125). Euel ethanol plants may have contingency plans to close if com prices rise to a certain level, eg, 3.50/bu or above (126). 

Concerning the reaction rate, a considerable decrease is observed qualitatively when R = C02Et or Ph. The presence of two Bu groups in the 3- and 5-positions (242 r3 = r5 = gyt) completely inhibits the reaction. Structure (248) for the transition state has been established from a kinetic study of the reaction between pyrazole and l-fluoro-2,4-dinitrobenzene <72JCS(P2)1420). 

---